1. Field of the Invention
The invention relates to a descrambling technique.
2. Description of the Related Art
To protect ownership rights and interests, television program providers usually scramble contents of pay-television programs, and provide only critical information required for a descrambling procedure to paid subscribers. Television program providers of different regions may employ scrambling mechanisms in different types and different levels. A currently prevalent Digital Video Broadcasting Common Scrambling Algorithm (DVB-CSA) specifies that a header of each image block needs to include two bits for indicating whether image contents carried in the image block are scrambled and for describing a key for descrambling.
To reinforce a protection level, television providers in certain regions scramble image blocks by alternatively utilizing several keys. For example, for a same video stream, the first one hundred image blocks are descrambled by a first key, the next one hundred image blocks are descrambled by a second key, the next one hundred image blocks are descrambled by the first key, and so forth. The television program providers then in advance provide the first key and the second key (which may be stored into a memory at a reception end) to a user. When descrambling at the reception end, the correct key is selected for descrambling according to the header contents of the image blocks.
To reduce production costs and inventory management complications, rather than manufacturing different chips for two different regions, a manufacturer of descrambling chips sometimes sells the same type of chips to two regions adopting different scrambling mechanisms. For example, it may be the case that a television program provider in a region A adopts a scrambling solution of utilizing one key, whereas a television program provider B adopts a scrambling solution of alternately utilizing two keys. A descrambling chip may be correspondingly designed to be capable of descrambling by alternately utilizing the two keys (for meeting requirements of region B), and descrambling by utilizing only one key with a part of the function (for meeting requirements of region A) turned off.
Current chip manufacturers mostly configure a descrambling mechanism of chips through software means according to a shipping destination of the chips. For example, part of the functionality is turned off for the descrambling chips to be sold to region A. Based on market considerations, a selling price of the descrambling chips sold to region A may be lower than the selling price of the same chip to region B. However, once the software for setting the descrambling mode in the chips is cracked or modified (e.g., the functions originally turned off by the chip manufacturer are turned on), it is possible that the descrambling chips intended to be sold to region A may be transported to and utilized in region B, thus causing considerable economic loss of the chip manufacturer.